Alkali lignin as a pH response bifunctional material with both adsorption and flocculation for wastewater treatment

Abstract Alkali lignin (AL) has attracted great attention as a material for treating dye wastewater due to its low cost and environmental friendliness. However, the unique structure and aggregation characteristics of AL regarding the dye wastewater removal mechanism have not been systematically revealed. Here, the removal process of typical cationic dye contaminants (methylene blue, MB) from dye wastewater by AL at different pH was explored and the unique synergistic effect mechanism of adsorption and flocculation was revealed. With increasing pH, the removal rate initially increased and then decrease. With increasing MB concentration, the optimal pH value corresponding to the maximum adsorption rate increased regularly. Zeta potential and Fourier transform infrared spectroscopy (FTIR) showed that electrostatic and π–π interactions and hydrogen bonding consisted push-pull balance under the influence of pH. In addition, scanning electron microscopy (SEM), ultraviolet and visible spectrum (UV) and particle size analysis showed that the aggregate structure and synergistic mechanism changed with the solution pH and concentration. In the low concentration solution, adsorption dominated. While in the high concentration solution, flocculation dominated. The removal mechanism consisted of the synergy of adsorption and flocculation laying the foundation for the efficient and environmentally friendly treatment of dye wastewater by AL.